Join Date: Jan 2019
Location: Sedgwick, Kansas
You can use any online CFM calculator to find out how many CFM you need based on RPM, cubic inches, and approximate volumetric efficiency. It's unlikely that a street 302 (or 306 in your case) will need a carb over 600 CFM. In fact, few 302's really need more than about 550 peak CFM, even if they're pretty healthy.
Having a 'larger' carburetor means less pressure drop in the intake (less vacuum at wide open throttle), which can make more power - to a point. When airflow going through the carburetor isn't enough to draw fuel from the boosters properly, you end up with problems. The type of booster being used also affects this. A typical Holley booster doesn't atomize the liquid very well, when it has low airflow. Big fat droplets of gas don't burn as well as a fine mist (or vapor), so your low and midrange performance suffers. Going to a "big" carb may offer a few more horsepower on the top end, but at the expense of throttle response, economy, and average power.
Using mechanical secondaries has much the same effect. Instead of feeding the engine what it needs, you're just dumping a big dollop of gas down the throat as you snap the blades open, and saying "I don't care what you want, just do it!" That can be okay for a manual transmission, especially on the track where you're not going to be using anything except for idle and WOT, but it isn't so great for an automatic in traffic.
While peak horsepower is king on the drag strip (usually), average horsepower means a lot more to any street driven car. Going with a carb that matches your engine, especially one with annular boosters that deliver a very fine mist of fuel will really help wake up your car, and give you better gas mileage to boot. Most of the Holley-based carbs (like the Demon, and even the Holley Dominator) offer annular boosters on some of their bigger offerings, but the Summit M-series carbs are a lot less expensive, and very effective. They come in different sizes starting at 500 and 600 CFM. For a mild build, the 500 would be great, but rounding up (to ensure more air than you will likely need) you'd want to go with a 600 CFM version.
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